It’s an icy cold December morning, but the site of the former Rebstockbad pool in Frankfurt’s Bockenheim district is bustling with activity. A huge, orange drilling rig towers over the crowd that has gathered for a press conference by the construction site. Journalists, state secretaries, lawyers from the commercial law firm FPS and employees from the city and state are among them. Why? Because deep underground, it is much warmer than up here – and not only on bitterly cold winter days.

The exceptionally high temperatures deep below Frankfurt have been common knowledge for some time. 100 metres below Frankfurt it is 28 degrees Celsius, compared to around 10 degrees elsewhere. Investigations by the Hessian Agency for Nature Conservation, Environment and Geology indicate that at a depth of 800 metres, thermal water flowing from the northern Upper Rhine Plain (Oberrheingraben) near Trebur to Frankfurt may even reach 40 degrees.

A building block for a climate-neutral Frankfurt 

The heat from the earth, a renewable energy source also known as geothermal energy, is a very handy tool for a city that intends to be climate-neutral by 2035. It is also equally suitable for heating, cooling and generating electricity. However, there are many geological factors to consider when extracting geothermal energy. It is thus not as easy to use as ‘plug and play’ solar energy, for example.

Test drilling at the Rebstockbad site began in November 2022 with the involvement of the city of Frankfurt am Main, the Hessian Agency for Nature Conservation, Environment and Geology and other project partners. The aim is to find out more about the temperatures, as well as the conductivity and permeability of the rock. The drill manages about ten metres per day, with a depth of 800 metres the target. The first results are expected to be available in March 2023. If all goes well, the new water park and the planned Römerhöfe residential area could be supplied with climate-friendly geothermal energy in the future.

Buried treasure 

The head of Frankfurt’s Climate department, Rosemarie Heilig, is looking forward to the results of the drilling. For her, one thing is clear: ‘We cannot remain dependent of fossil fuels and other countries.’ She believes geothermal energy will be essential to generate sufficient renewable energy within the confines of Frankfurt. That’s why Heilig emphasises: ‘If we can prove with the research pilot hole that a treasure trove of heat worth exploiting lies dormant under Frankfurt, then we will certainly make use of it.’ Moreover, Julia Woth from LandesEnergieAgentur Hessen (LEA) emphasises the particular advantages of geothermal energy: it is available regardless of weather conditions and political crises – reliably, at a stable price and in a climate-friendly manner.

State Secretary Jens Deutschendorf from the Hessian Ministry of Economics also has high hopes for geothermal energy and the pilot hole. He describes it as a key project for the city. ‘With its year-round availability, geothermal energy is an excellent complement to sun and wind and is particularly interesting for heating buildings,’ he emphasises. ‘Time and again, there are calls to reduce CO₂ emissions as quickly as possible. The solution could be under our very feet.’

Climate-friendly pool days 

Dr Boris Zielinski, head of the Sports department and in charge of the Frankfurt swimming pool companies, is also eager to see the results: ‘If the use of geothermal energy is successful, the new Rebstockbad will also be a lighthouse project for the energy concept of all swimming pools in Germany.’

The pilot borehole at the Rebstockbad site is a medium-depth borehole, but it is the deepest one ever drilled in the Frankfurt metropolitan area. There are a further 280 geothermal energy projects in Frankfurt, for example on the site of the Henninger Tower and the FOUR Frankfurt high-rise ensemble, which is still under construction. Drilling is also underway at 20 other locations in Hesse.

An energy source with tradition 

Underground heat sources have been known of for some time, explains Professor Thomas Schmid, President of the Hessian Agency for Nature Conservation, Environment and Geology: ‘The first drilling in Frankfurt took place as early as 1893,’ he says. Back then, a brewery from the Sachsenhausen district drilled to a depth of 286 metres and discovered thermal water at a temperature of 30 degrees Celsius. But, at that time, they didn’t know what to do with it, and filled the hole in again. ‘The water was too warm to brew beer,’ says Schmid, adding with a smile: ‘Frankfurt could have become a spa town.’ 

Legal support

Geothermal energy offers many opportunities. But a project as complex as the pilot hole drilling at the Rebstockbad site also brings legal challenges. That is why the Frankfurt-based commercial law firm FPS is advising LEA on the matter. They are proud to be involved in such an important project for the future and glad to gain an insight into the world of geothermal energy, say FPS lawyers Dr Annette Rosenkötter and Dr Olaf Dziallas during the on-site visit.

Rosenkötter advises LEA on public procurement law: ‘One of my tasks is to ensure that the tenders and contracts awarded to the construction companies involved are legally sound.’ Due to the high level of time pressure and the small number of drilling companies around, this was sometimes challenging, but also exciting, she adds. 

Her colleague, Dr Dziallas, provides LEA with support regarding administrative, construction and liability law. ‘The theoretical risks are small, but diverse: regardless of whether an excavator tips over during a storm or perhaps water unexpectedly shoots out of the ground at high pressure – it must be clear to those involved, and especially to the insurance company, who is liable.’

Of course, all parties involved hope that there will be no such incidents and that the rock under ‘Mainhattan’ is permeable enough for sufficient quantities of hot water to be extracted. Then, in the near future, no one will have to freeze on the Rebstockbad site anymore, even on cold winter days. 

Text: Silke Bauer
Images: Jens Lindemann